FD radios suffer from Self Interference (SI), a term referring to the signal transmitted by the FD radio TX that leaks to the FD radio RX. At the RX of the FD radio, the SI power can be many times stronger than the power of the received signal of interest. Consequently, SI can severely degrade the reception of the signal of interest, and thus SI mitigation is required in order to maximize the spectral efficiency gain of the FD operation. As the number of antennas increases, mitigating SI becomes more challenging, since more antennas naturally result in more SI components. Conventional SI suppression techniques in Single-Input Single-Output (SISO) systems include propagation domain isolation, analog domain suppression, and digital cancellation[1]}, [2]}.
Although analog SI cancellation in FD MIMO systems can be implemented through SISO replication, its hardware requirements scale with the number of TX/RX antennas. The authors in [3]}, [4]} presented spatial suppression techniques that alleviate the need for analog SI cancellation, which was replaced solely by digital TX/RX beamforming. In [5]}, a joint design of multi-tap analog cancellation and TX/RX beamforming, where the number of taps does not scale with the product of TX and RX antenna elements, was proposed.
The FD technology has been lately theoretically combined with Hybrid analog and digital BeamForming (HBF) [6]} to enable simultaneous UL and DL communications in massive MIMO systems operating in the millimeter wave frequency bands [7]}, [8]}, [9]}, [10]}. These works mainly assigned the role of SI mitigation to the hybrid beamformers and/or deployed analog SI cancellation that scales with the number of TX/RX antennas.
